ASSOCIATIVE FACTORS UNDERLYING THE PIGEON'S KEY PECKING IN AUTO-SHAPING PROCEDURES'

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1 JOURNAL OF THE EXPERIMENTAL ANALYSIS OF BEHAVIOR 1973, 19, NUMBER 2 (MARCH) ASSOCIATIVE FACTORS UNDERLYING THE PIGEON'S PECKING IN AUTO-SHAPING PROCEDURES' ELKAN R. GAMZU AND DAVID R. WILLIAMS UNIVERSITY OF PENNSYLVANIA Key pecking in pigeons can be engendered by associating response-independent food presentations with illumination of a key. Specific pairings of key and food are not necessary for this phenomenon. Differential positive association between key and food (defined in terms of relative densities of reinforcement), however, is necessary and sufficient to produce and maintain key pecking. Thus, the occurrence of key pecking in auto-shaping can be considered to depend on associative processes similar to classical conditioning. Consequently, auto-shaped pecking can be virtually eliminated by the addition of food presentations in the intertrial interval, thus removing the association between key and food. Initial exposure to random reinforcement, or reinforcement only in the absence of the key, results in lower rates of pecking in subsequent auto-shaping procedures. In previous studies of auto-shaping in pigeons (Brown and Jenkins, 1968; Williams and Williams, 1969) the procedures resembled the Pavlovian delay conditioning paradigm. Specifically, the transillumination of a standard pecking key was followed a few seconds later by the presentation of a grain reinforcer. Gamzu and Williams (1971) explored the role of the stimulus reinforcer in the autoslhaping effect by attempting auto-shaping with a procedure that differed markedly from the Pavlovian delay conditioning procedure of Brown and Jenkins. Gamzu and Williams' basic procedure is illustrated in Figure lb. An experimental session is composed of two portions, distinguished on the basis of key illumination. Once each second throughout the course of the experimental session a probability generator is sampled and determines whether or not, in that second, a reinforcing event (the 4-sec presentation of grain) will be initiated. In the differential condition of Fig- 1This paper is based on a dissertation submitted by the first author to the Faculty of the Graduate School of Arts and Sciences, The University of Pennsylvania, 1971, in partial fulfillment of the requirements for the Ph.D. degree. The research was supported by Grant G14055 fromii the National Science Foundation. The authors thank N. Adler and R. L. Solomon for appraisal of an earlier version of the nmanuscript. We are grateful to Tom Allaway, Joe Bernheim, Barry Schwartz, Alan Silberberg, Harriet Williams, and Kit Zonana for their perceptive comtmients. Reprints may be obtained from Elkan Gamzuz, Pharmacology Department, Hoffmann-La Roche Inc., Nutley, N. J ure 1 B, the probability of initiating a reinforcement is zero when the key is not illuminated and greater than zero (in our work 0.03) during periods of key illumination. In the FOOD FOOD --- A. AUTO-SHAPING I /sc. PG SAMPLE B. DIFFERENTIAL FOOD I /sec. PG SAMPLE C. NON-DIFFERENTIAL FOOD - I /sec. PG SAMPLE D. DIFFERENTIAL-ABSENCE FOOD E. NO-REINFORCEMENT Fig. 1. Schematic representation of (a) the basic autoshaping paradigmii and (b-e) the procedures used here, in which food presentation was determined by a positive outcome of a one-per-second probability generator sample. (b) The differential condition in which food was presented randonmly in time, but only during illuminated key trials. (c) The non-differential condition in which food was presented randomly in time during trial and intertrial intervals. (d) The differential-absence condition in which food was presented only in the intertrial interval. (e) The no-reinforcement condition in wvhich food was never presented. 225

2 226 ELKAN R. GAMZU and DAVID R. WILLIAMS non-differential condition of Figure IC, the probability of reinforcement is constant and independent of key illumination. Under the differential procedure, the key is positively correlated witlh a difference in the average frequency of reinforcement, but its illumination is in no way related to specific instances of reinforcement presentation. The situation contrasts with the auto-shaping or delay conditioning paradigm shown in Figure IA, where onset of key illumination is specifically correlated with a subsequent time of presentation of a reinforcer. Because the period of key illumination is correlated only with the average frequency of reinforcement and is not related to specific occurrences of reinforcement, we feel this paradigm is at once significantly different from delay conditioning paradigm of conventional auto-shaping, and also closer to conditions that often prevail in free-operant experiments using schedules such as variable interval (cf. Gamzu and Schwartz, 1973). The paradigm of Figure 1B has been shown to be effective in producing secondary reinforcers and in establishing conditioned suppression (Egger and Miller, 1962, 1963; Rescorla, 1968). It has also been shown to be effective in establishing auto-shaping of the pigeon's key peck (Gamzu and Williams, 1971). The present study sought to explore more fully the properties of the stimulus-reinforcer association as they apply to auto-shaping. EXPERIMENT 1 The purpose of Experiment 1 was to contrast performance under the differential and non-differential procedures illustrated in Figure lb and C in an auto-shaping situation. Both procedures were applied to individual birds, beginning with either the differential or the non-differential procedure, in order to assess the reversibility of these conditions. METHOD Subjects Twenty experimentally naive adult male Silver King pigeons were maintained at 80'/ of their free-feeding weight. Apparatus All experiments were conducted in a standard Lehigh Valley pigeon chamber measuring 11 by 11 by in. (28 by 28 by 26 cm). Only the center pecking disk was used. This disk was 7.5 in. (19 cm) above the floor of the compartment and 4.5 in. (11 cm) above the grain hopper and could be transilluminated by white light. Automatic scheduling and recording equipment were located in a separate room. Procedure At the beginning of the first session, pigeons were trained to approach rapidly and eat from the hopper whenever it was presented. Food presentation involved 4-sec access to mixed grain. A daily session was composed of 50 trials of disk illumination for 8.6 sec. The intertrial interval varied according to a geometrical progression with a mean of 30 sec (range 10 to 120 sec). Grain presentations were always independent of the pigeon's behavior and pecking had no effect on the experimental procedure, the only consequences of key pecking being the slight auditory feedback of the microswitch clostire. As shown in Figure 1, grain presentations were determined by sampling from a probability unit once every second. The probability generator was set at p = 0.03, but its output could be gated out during the illuminated key presentations, the intertrial interval (ITI), or during both. This generated four different procedures. In the differential condition, which was the basic testing procedure, grain was presented only during the illuminated key presentations but never in the ITI. In the non-differential, or random, procedure grain was presented with an equal density both in the presence or absence of illumination of the pecking disk. The no-reinforcement, or extinction, procedure merely involved the presentation of illuminated key trials without food. The differential-absence condition was generated by presenting food only in the ITI, so that the trials were differentially associated with the absence of food. Each of these procedures can be summarized by a pair of conditional probabilities; namely, the conditional probability of grain presentation in each second given illumination of the key, and the conditional probability of grain presentation given absence of illumination of the key. Table 1 gives a summary of the procedures in terms of these conditional probabil-

3 ASSOCIATIVE CONTROL AND AUTO-SHAPING 227 ities. In the first experiment, only the differential and non-differential procedures were used. The other two procedures were used in Experiment 2. Table 1 The conditional probabilities of food presentation during each second in the presence and absence of the illuminated key, for the four basic procedures used in this study. P (FOOD/) P (FOODi) Differential Non-Differential Differential-Absence No Reinforcement Eleven of the pigeons were given 35 days exposure to the differential condition-group Diff. On the thirty-sixth day, seven of these pigeons were given an additional 14 days on the non-differential procedure, followed by 10 more sessions of the differential procedure. The other nine birds (Group Non-diff) received initial exposure to the non-differential procedure for 14 days, following which all received 35 days on the differential procedure. RESULTS Results from the first 35 sessions of exposure to the differential procedure are shown in the top curve of Figure 2, which presents the mean rate of responding per minute of key illumination (plus or minus one standard error) for the 11 birds of the Diff group. Although there were individual differences in the rate at which auto-shaped responding was acquired, all pigeons eventually pecked at the key with high constant rates. On the last day of testing (Session 35) the range of response rates was from 50 to 130 responses per minute; only one pigeon showed a rate lower than 60 responses per minute. Response rates were always calculated for illuminated trial periods only. The few responses that occurred during the intertrial interval occurred immediately after a trial offset. It is to be noted that these high rates of responding developed even though pecking had no scheduled effect on the delivery of the grain reinforcer. The Non-diff group of nine pigeons was initially exposed to the non-differential procedure under which reinforcement was presented with equal probability throughout the SESSIONS Fig. 2. Acquisition of key pecking when food was randomly presented only during Illuminated key trials. Mean daily response rates (± one S.E.) during the key presentations only for two groups. Group Diff (n = 11) had no pretreatment. Group Non-Diff (n = 7; two birds are omitted-see text for explanation) had previously been exposed to 14 days of random reinforcement during both the trial and the intertrial intervals. session. A total of 30 responses was recorded for all nine birds during the 14 days of this treatment. These were evenly distributed between trials and ITI periods and were not maintained. After 14 days of exposure to this condition, the differential procedure of Figure lb was instituted. The lower curve of Figure 2 shows the mean rate of responses per minute of key illumination shown by seven of the nine birds in this group. The two pigeons whose data are not included in the curve pecked at high rates similar to those maintained by the differential group. The seven birds shown responded at considerably lower rates even after extended exposures (35 sessions) to the differential condition. In Session 35 of the differential procedure, the range of response rates was 10 to 30 responses per minute. A statistical analysis of mean response rate on the differential procedure (including the two pigeons whose high-rate data are omitted from the figure) shows a significant difference in mean response rate between pigeons in the two groups (data from Session 20, t = 2.61, df = 18, p < 0.02, two-tailed). Including observations taken on new birds under conditions identical to those of the non-differential group, we have found that 13 of 15 pigeons so tested show low rates of responding when the differential procedure is put in force after 14 days exposure to the non-differential procedure.

4 228 ELKAN R. GAMZU and DAVID R. WILLIAMS co 4L) a 0 Co 0 0 c0 10 minutes %~~b c d Fig. 3. Representative cumitulative records of pecking duiring key presentations, wvhen food wtas randomly presented duiring key-trial presentations. (Twenty-ninth day of the differential procedure.) The cumulative recorder wvas operative only during the trials. Records a and b show the high constant rates observed when pigeons were introduced to the procedure experimentally naive (Group Diff). Records c and d show the lowv irreguilar rates observed wvlhen pigeons w-ere first exposed to random food presentations in both trial and intertrial intervals for 14 days (Group Non-Diff). Typical response rates and patterns of responding can be seen in Figure 3, which presents cumulative records of responding to the illuminated key on the twentietlh-ninth day of the differential procedure for fotur pigeons. The cumulative recorder was turned off during the intertrial interval. The records marked a and b are for two birds in the Diff group, and show a typical higlh constant rate of responding similar to that maintained on conventional VI schedules (Ferster and Skinner, 1957). Records c and d are for two pigeons whose initial exposure was to the non-differential procedure. These rates are clearly low and not especially constant. Folloving the 35 sessions of Figure 2, seven of the pigeons in the Diff group were then exposed to the non-differential condition. As slhown in Figure 4, once periods of key illumination were not correlated with a difference in rate of reinforcement, a precipitous drop in response rate was observed. On the last day of the non-differential procedure, two of the seven pigeons made no responses, two made one response each, and the other three made 5, 31, and 53 responses. Figure 4 also shows the effect of reinstating the differential procedure after this training. There was a rapid recovery of high-rate performance and complete recovery of the levels of responding that were sustained on the original differential proce- 5x IL Im 50 z Pro SESSIONS Fig. 4. Reduction of established key pecking when food is presented randomiily in both trial and intertrial intervals (non-differelntial proceclure) and reacquiisition wvhen food is presented only in trial intervals (differential procedure). Daily mean response rate (± one S.E.) during key presentations for seven pigeons. dure. These effects can be seen in the right hand portion of Figure 4. Results from the differential group confirm the previous report that a differential in reinforcement rate in the presence of an illuminated key is sufficient to establish auto-shaping, that the non-differential procedure results in a rapid and nearly complete cessation of responding within a few sessions, and that reinstatement of the differential procedure rapidly reinstates the original pattern of responding after 14 days of exposure to an intervening non-differential condition. Qualitatively, results from the group initially exposed to the non-differential procedure were similar: under the non-differential procedure, virtually no responding was observed, and after 14 days exposure to the non-differential procedure, instating the differential procedure led to the development of consistent responding. Inspection of Figure 2 indicates that responding in the differential condition

5 ASSOCIATIVE CONTROL AND AUTO-SHAPING 229 showed a similar rate of development in both groups, even tlhough the level ultimately obtained was considerably lower. Experiment 1 suggests that initial exposure to the non-differential condition does not interfere qualitatively with auto-slhaping nor with subsequent sustained responding, but does have a marked quantitative effect: the rates of responding attained under the differential procedure are far lower if the bird's initial exposure is to the non-differential procedure. The effect is not entirely determinate, because two of the birds originally exposed to the non-differential procedure subsequently developed typical high rates of responding. The quantitative effect is a substantial majority of all birds tested was unanticipated, however, and represents a surprising non-reversibility of the order of experimental treatments. The effect is all the more surprising because of the rapid recovery of high rate responding by the differential group after a period of 14 days exposure to the non-differential condition. It appears that the inital experimental condition is critical in determining whetlher higlh or low rates will develop. Experiment 2 was carried out in an effort to delineate more sharply the critical variables involved. EXPERIMENT 2 Since pecking at no time influenced the presentation of reinforcement in Experiment 1, it is obvious that the difference in response rate between the two groups is not to be found in any explicit response-reinforcer relationship. Furtlhermore, it seems clear that the initial conditions of exposure are critical to the effect: no bird initially exposed to the differential procedure developed a low rate. Lowv rates of responding were produced only when the non-differential procedture came first: when it was interposed between an original and a subsequent differential condition low rates of responding did not develop. What is the critical feature of initial exposure to the non-differential condition? One possible factor might be the "informativeness" of the key, that is, whether or not in the sessions wlhere it was originally presented, it signalled something about reinforcement density. A second possible factor might be response-related: low rates might arise if a bird's initial experience in the sittuation involved many hours of non-responding. If either of these possibilities were operative, then low rates of responding should arise if 14 sessions of training without any reinforcement were administered before the differential condition was introduced (Figure IE). Alternatively, low rates might arise due to some effect of extended presentations of reinforcement in the absence of the key. This possibility can be investigated by exploring the effects of the differential-absence condition illustrated in Figure ID. In this condition, the key is "informative" in the sense that it signals a period of non-reinforcement; in the absence of key illumination, reinforcement is received at the same rate as it had been in the nondifferential procedure of Figure 1C. Experiment 2 sought to explore these possibilities by initially training birds under the differential-absence procedure and the no-reinforcement procedure, and then exposing them to the differential procedure of Figure 1B in order to isolate the factor responsible for the low rates of responding observed in the non-differential group of Experiment 1. These conditions also help to delineate more precisely the stimulus-reinforcer relationships necessary for auto-shaping. METHOD Stubjects Six experimentally naive adult male Silver King pigeons were maintained at 80% of their free-feeding weight. A pparatus The same as used in Experiment 1. Proceduire Four pigeons were assigned to Group Noreinf, which received 14 days on the no-reinforcement condition. The trials were presented as usual but no reinforcement was given. On Day 15, the procedure was changed to the differential condition described above, for another 21 days. The other two pigeons were in Group Diff- Abs. This group received 14 days of initial exposure to the differential absence procedure in which reinforcement was presented only in the ITI. On Day 15, the condition was changed to the differential procedure for a further 35 days.

6 230 ELKAN R. GAMZU and DAVID R. 60 r WILLIAMS z U) 2 w 30- (0 w z 0. R o w., I 0 x~~~' or 0)I.XI X~~~~~~~~~~~~~~~' X. 50II SESSIONS Fig. 5. Acquisition of key pecking when food was randonmly presented only during illuminated key trials for three pigeons in the No-Reinf Group, which had previously been exposed to 14 days of trial presentations without any food delivery. Response rates are for trial intervals only. RESULTS Only one of the six pigeons pecked the key at all during the first 14 days of the experiment. This pigeon in the No-reinforcement group made a total of 16 pecks at the key all of which occurred during the ITI when the key was darkened. When the pigeons in the No-reinforcement group were subsequently exposed to the differential condition, they began pecking at the key and developed high rates of responding. Figure 5 shows individual acquisition curves for three of the birds. These three birds all developed response rates in the high range similar to those observed in pigeons in the Diff group of Experiment 1, where initial exposure was to the differential condition. The fourth bird in the No-reinforcement group made few responses, averaging only two per session for the last 24 days. This rate of responding is at least an order of magnitude lower than has been observed for any other pigeon under this procedure (regardless of prior experience) and these data are therefore considered anomalous. i I to IS SESSIONS Fig. 6. Acquisition of key pecking when food was randomly presented only during illuminated key trials for two pigeons in the Diff-Abs Group, which had previously been exposed to 14 days in which food was presented only during the ITI. Response rates are for trial intervals only. Treating only the data from the three birds that developed high rates, this group does not differ statistically from the diff group of Experiment 1: (Session 20 analyzed t = 0.47, df = 12, p < 0.60). However, the group does differ in mean rate from the non-diff group of Experiment 1 (data from day 20, t = 1.81, df= 10, p<0.10). The two pigeons whose initial exposure was to the differential-absence condition also pecked at the key when exposed to the differential condition. Figure 6 shows the acquisition of pecking for these two birds in terms of mean daily response rate to the illuminated key during the 35 days of this phase of the experiment. The response rates are clearly in the low range and remain so for the duration of testing. These results are similar to those obtained in the Non-diff group of Experiment 1 when those birds were subsequently exposed to the differential condition. It seems, therefore, that the low rates of response observed in the Non-diff group of Experiment 1 were the result of extensive reinforcement during the intertrial interval in the initial procedure experienced. The high rates of responding observed in the no-reinforcement group are sufficient to rule out the possibility that low rates arise from extended exposure to the situation without the development of pecking or to initial exposure to a non-informative key. DISCUSSION The major finding of this experiment is that auto-shaped key pecking developed only if il-

7 ASSOCIATIVE CONTROL AND AUTO-SHAPING lumination of the key was positively associated with a difference in the average frequency of reinforcement. The effectiveness of a procedure that is different from the original delay conditioning paradigm in a number of particulars indicates that a strict pairing of key and food is not necessary for auto-shaping to occur. The failure of the non-differential procedure to develop or sustain previously established responding argues against an attribution of sustained rates of responding to sporadic feeding in the presence of an illuminated key, or to a tendency to peck illuminated keys in a situation where food is sometimes available. More fundamentally, the failure of the nondifferential condition to establish or sustain responding suggests that a positive correlation or association between the stimulus of key illumination and some parameter of reinforcement (for example, frequency) is a necessary condition for both developing and maintaining auto-shaped key pecking. It is noteworthy that the rapid cessation of responding under the non-differential condition in Experiment 1 took place not because reinforcement was withheld, but because reinforcement was added to the experimental situation. When the nondifferential procedure was introduced to birds already responding at a high rate, there was no operational change in conditions in force during periods of key illumination: what was altered were the conditions in force when the key was not illuminated. It does not seem possible to explain this cessation of responding by appeal to events taking place solely within the confines of the period of key illumination itself; rather the dual relationship of the key to reinforcement parameters in its presence and in its absence must be taken into account. These findings are in accord with Rescorla's "contingency" analysis of classical conditioning (1967), according to which it is not an association of stimulus and reinforcer per se that results in conditioning, but rather the correlation of the stimulus with occasions of reinforcement and non-reinforcement. Thus, the similarity between auto-shaping and Pavlovian conditioning is not limited to procedure, but is also seen in the behavioral outcomes. The pecking that develops to the key likewise seems analogous to other "anticipatory" conditional responses observed in delay conditioning. The strong analogy between procedure and behavioral outcome suggests that classical conditioning is in some way intimately involved in the auto-shaping phenomenon. What is the role of classical conditioning in auto-shaping? The present results, along with others (Brown and Jenkins, 1968; Williams and Williams, 1969; Allaway, 1971) suggest that auto-shaping depends on the "informativeness" of key illumination with respect to reinforcement. It seems likely to propose that the stimulus-reinforcer association, correlation, "information", or "contingency" that is effective in auto-shaping is paradigmatically the same as the correlation that is effective in classical conditioning using other stimuli and reinforcers. All studies of auto-shaping of which we are aware support this view. On the basis of the present work, however, we cannot speculate on the other aspect of respondent involvement, namely the determination of the response class itself. Although that aspect is beyond the reach of this paper, we do believe that the assertion of a common S-S associative involvement between auto-shaping and classical conditioning is a substantive step. It suggests, for example, a need for experimental analysis of auto-shaping situations with other species and manipulanda (e.g., Sidman and Fletcher, 1968) to ascertain whether autoshaping is by pseudo-conditioning or the associative relations of classical conditioning. The significance of this distinction would be most fully seen when sustained maintenance or even negative automaintenance was attempted (see Gamzu, 1972). The strong effect of order-of-experimental conditions was an unanticipated outcome of the present study. It is particularly puzzling because it apparently occurs only in the case where birds are given a period of initial exposure to the non-differential procedure: initial exposure to the differential procedure can apparently eliminate the low-rate pattern that follows exposure to the non-differential condition. Two features of the low-rate responding seem particularly notable: first, the onset of key pecking as such does not seem conspicuously delayed relative to the group that develops high-rate reponding. Indeed, a comparison over both experiments of the number of trials to the first peck for pigeons that ultimately develop low rates of pecking versus pigeons that developed high rates of pecking showed no hint of a reliable difference in the time of first peck (Ward-Wolfowitz runs test:

8 232 ELKAN R. GAMZU and DAVID R. WILLIAMS runs = 15, p < 0.42, see Siegel, 1956). Secondly, once responding was established it was maintained over a number of sessions without further signs of increase, even though the same opportunities for the development of high rates of pecking through "adventitious reinforcement" were present under these conditions that were present under conditions where birds developed high rates of responding. Given the fact that substantial rates of responding were established (the mean rates of about 20 responses per minute are relatively "low" but nevertheless substantial), it is surprising that lengthy exposure to the non-differential condition did not obliterate the low-rate response pattern, even after the thirty-fifth session of exposure. We hope this paper will encourage the analysis of situations where behaviors are nonarbitrarily related to reinforcers, and where correlations among stimuli, responses, and reinforcers are unambiguous but not necessarily precise. A tight and controlling response-reinforcer relationship may be necessary in situations involving an arbitrary operant, in order to determine the form of the response, but such a stricture is not necessary where introduction of the behavior takes place non-arbitrarily, as a consequence of the reinforcer itself (see also Staddon and Simmelhag, 1971). In that case, correlations between responding and overall reinforcement frequency (see Herrnstein and Hineline, 1966) and stimuli and reinforcement (see Gamzu and Schwartz, 1973), may be sufficient to provide powerful control over behavior, and to provide avenues of behavioral control previously overlooked. In this connection, it is noteworthy that the "non-differential" reinforcement situation dramatically reduced the level of pecking (an effect that resembles extinction through the addition of reinforcers). Our direct conclusion is that responding weakened because the associative control developed by the key was removed. A more general exploration of this form of reducing or eliminating behavior may be indicated, however. Where it is impractical to remove a reinforcer entirely, as in some natural or applied situations, circumstances may be found where the uncorrelated addition of reinforcing events will reduce the frequency of the response they sustained when the correlatio-n-either stimulus-reinforcer or response-reinforcer-was stronger. REFERENCES Attention, information, and auto-shzap- Allaway, T. A. ing. Unpublished Ph.D. Dissertation, University of Pennsylvania, Brown, P. L. and Jenkins, H. M. Auto-shaping of the pigeon's key-peck. Journal of the Experimental Analysis of Behavior, 1968, 11, 1-8. Egger, D. and Miller, N. Secondary reinforcement in rats as a function of information valuie and reliability of the stimulus. Journal of Experimental Psychology, 1962, 64, Egger, D. and Miller, N. When is a rewvard reinforcing?: an experimental study of the information hypothesis. Journal of Comparative and Physiological Psychology, 1963, 56, Ferster, C. B. and Skinner, B. F. Schedules of reinforcemzent. New York: Appleton-Century-Crofts, Gamzu, E. Auto-shaping, and the failure of automaintenance with a negative response-reinforcer contingency in squirrel monkeys. Paper presented at the SEPA Meeting, Atlanta, Georgia, Gamzti, E. and Schwvartz, B. The maintenance of key pecking by stimulus-contingent and response-independent food presentation. Journal of the Experimental Analysis of Behavior, 1973, 19, Gamzu, E. and Williams, D. R. Classical conditioning of a complex skeletal response. Science, 1971, 171, Herrnstein, R. J. and Hineline, P. N. Negative reinforcement as shock-frequency reduction. Jouirnal of the Experimttental Analysis of Behavior, 1966, 9, Rescorla, R. A. Pavlovian conditioning and its proper control procedures. Psychological Review, 1967, 74, Rescorla, R. A. Probability of shocks in the presence and absence of the CS in fear conditioning. Jouirnal of Comparative and Physiological Psychology, 1968, 66, 1-5. Sidman, M. and Fletcher, F. G. A demonstration of auito-shaping with mlonkeys. Jouirnal of the Experimnental Analysis of Behavlior, 1968, 11, Siegel, S. Nonparamtietric statistics for the behavioral sciences, Newv York: McGrawv-Hill, Staddon, J. E. and Simmelhag, V. L. The. "superstition" experimient: a reexamination of its imiiplications for the principles of adaptive behavior. Psychological Review, 1971, 78, Williams, D. R. and Williams, H. Automaintenance in the pigeon; sustained pecking despite contingent non-reinforcement. Journal of the Experimental Analysis of Behavior, 1969, 12, Received 7 January (Final acceptance 12 September 1972.)